📄 rfc3094.txt
字号:
Sprague, et al. Informational [Page 11]
RFC 3094 Tekelec's Transport Adapter Layer Interface April 2001
Some of the facts concerning the TALI protocol which are important to
understanding how TALI works that are not evident from Figure 3
include the following:
* Each TALI connection is provided over a single TCP socket.
* The standard Berkeley sockets interface to the TCP is used by
the TALI layer to provide connection oriented service from
endpoint to peer endpoint.
* TCP sockets are based on a Client/Server architecture; one end
of the TALI connection must be defined as the 'server side',
the other end is a 'client'.
* The client/server roles are important only in bringing up the
TCP connection between the 2 endpoint, once the connection is
established both ends use the same Berkeley sockets calls
(send, recv) to transfer data.
* The TCP socket must be connected before the 2 TALI endpoints
can begin communicating.
* TALI provides user control over each TALI connection that is
defined. This control:
* Allows the user to control when each TALI connection will be
made
* Allows the user to control when each TALI connection is allowed
to carry SS7 traffic
* Allows the user to control the graceful shutdown of each socket
* TALI provides Peer to Peer messages. These messages originate
from the TALI layer of one endpoint of the connection and are
terminated at the TALI layer of the other endpoint. Peer to Peer
messages are used:
* To provide test and watchdog maintenance messages
* To control the ability of each socket to carry SS7 service
messages
* TALI provides Service messages. These messages originate from the
layer above the TALI layer of one endpoint of the connection and
are transferred to and terminated at the layer above the TALI
layer of the other endpoint.
Sprague, et al. Informational [Page 12]
RFC 3094 Tekelec's Transport Adapter Layer Interface April 2001
* The service messages provide several different ways to
encapsulate the SS7 messages (SCCP/TCAP, ISUP, and other MTP3
layer data) across the TCP/IP connection.
* As we will see later, different Service opcodes are used to
communicate across the TALI socket exactly how each SS7 message
has been encapsulated.
* A set of TALI timers is defined. These timers are used to
correctly implement the TALI state machine.
2.3.1 An Alternate TALI Protocol Stack using the SAAL Layer
This section presents a different, slightly more complex, TALI
protocol stack that can be used in place of the protocol stack in the
previous section.
Figure 3 in the previous section provided a simple illustration that
highlighted the basic TALI protocol stack that can be used to
transport SS7 MSUs between 56 Kbps links on the SS7 side of an SG and
the IP devices.
Figure 4 below illustrates an alternate TALI protocol stack that
includes the SAAL layer as part of the data transferred across the
TCP/IP connection.
Sprague, et al. Informational [Page 13]
RFC 3094 Tekelec's Transport Adapter Layer Interface April 2001
SS7 traffic SS7 traffic
via DS1 links via TALI
+-----------+ +----+ +--------+
|Traditional| | SG | | IP |
|SS7 Devices|<------>| |<-------->| Devices|
+-----------+ +----+ +--------+
SS7 DS1 SS7, TALI, TCP/IP
protocol stack protocol stack
+-----------------+ +-----------------+
| SS7 application | | SS7 application |
| layer | | layer |
+--------+--------+ +--------+--------+
| TCAP | ISUP | | TCAP | ISUP |
+--------+ | +--------+ |
| SCCP | | | SCCP | |
+--------+--------+ +--------+--------+
| MTP3 | | MTP3 |
+-----------------+ +-----------------+
| SAAL | | SAAL |
|(SSCF,MAAL,SSCOP)| |(SSCF,MAAL,SSCOP)|
+-----------------+ +-----------------+
| AAL5 | | TALI |
+-----------------+ +-----------------+
| ATM | | TCP |
| (& phy. | +-----------------+
| layer) | | IP |
+-----------------+ +-----------------+
| MAC |
| (& phy. |
| layer) |
+-----------------+
Figure 4: An Alternate TALI Protocol Stack with SAAL
The following bullets provide a discussion regarding the differences
between these 2 protocol stacks, the reasons for having 2 protocol
stacks, and the advantages of each:
* When the TALI protocol stack is implemented without the SAAL
layer, as in Figure 3, the SEQUENCE NUMBER of the SS7 MSU is NOT
part of the data transferred across the TCP/IP connection. In 56
Kbps SS7 links, the MTP2 header contains an 8 bit sequence number
for each MSU. The sequence number is used to preserve message
sequencing and to support complex SS7 procedures involving MSU
retrieval during link changeover and changeback. As indicated in
Figure 3, the MTP2 header is NOT part of the data transferred
Sprague, et al. Informational [Page 14]
RFC 3094 Tekelec's Transport Adapter Layer Interface April 2001
across the TCP/IP connection. The TALI protocol stack without
SAAL still guarantees correct sequencing of SS7 data (this
sequencing is provided by sequence numbers in the TCP layer),
however that protocol stack can not support SS7 changeover and
changeback procedures.
* When the TALI protocol stack is implemented with the SAAL layer,
as in Figure 4, the SEQUENCE NUMBER of the SS7 MSU IS part of the
data transferred across TCP/IP. In SS7 DS1 links, the SSCOP
trailer contains a 24 bit sequence number for each MSU. This 24
bit sequence number serves the same purposes as the 8 bit SS7
sequence number. As indicated in Figure 4, the SSCOP trailer IS
part of the data transferred across the TCP/IP connection. The
protocol stack in Figure 4 can support SS7 changeover and
changeback procedures.
* Implementing the TALI protocol with SAAL therefore provides
support for SS7 co/cb and data retrieval and can help to minimize
MSU loss as SS7 links are deactivated. However, implementing SAAL
is not a trivial matter. The SAAL layer consists of 3 sublayers
(SSCF, SSCOP, and MAAL), one of which (SSCOP) is quite involved.
It is envisioned that most SS7 to TCP/IP applications will NOT
choose to implement SAAL.
2.3.2 An Alternate TALI Protocol Stack using SCTP
The TALI protocol is dependent on a reliable transport layer below
it. At the initial design of TALI, TCP was the only reliable, proven
transport layer. Simple Control Transport Protocol (SCTP) is
currently being designed as a transport later specifically for
signalling. Once SCTP is a proven and accepted transport protocol,
SCTP can then be used in place of TCP as shown in Figures 3 and 4.
2.4 Inputs to the TALI Version 1.0 State Machine
Figure 5 illustrates the inputs that affect the TALI State Machine.
Inputs to the state machine include:
* Management events (ie: requests from the human user of the TALI
connection) to control the operation of a particular TALI session.
* TALI messages received from the Peer. These messages include peer
to peer messages as well as service data messages.
* Events from the User of the TALI layer. The user is the layer
above TALI in the protocol stack, either the SS7 or SAAL layer.
Sprague, et al. Informational [Page 15]
RFC 3094 Tekelec's Transport Adapter Layer Interface April 2001
* Implementation Dependent Events. Each implementation must provide
inputs into the TALI state machine such as:
* Socket Events
* TALI protocol violations. The TALI state machine must detect
protocol violations and act accordingly.
* Timer events.
Sprague, et al. Informational [Page 16]
RFC 3094 Tekelec's Transport Adapter Layer Interface April 2001
+====+ +============+
| | +---------+ +-------------+ | |
|User| | Service | | Mgmt. Open | | MANAGEMENT |
|Part|<-->| Message | | Mgmt. Close |<-->| |
| | | | | Mgmt. Proh. | | |
| | +---------+ | Mgmt. Allow | +============+
+====+ ^ +-------------+
| ^
| |
v v
+========================================================+
| TALI State Machine |
+========================================================+
^ ^ ^ ^
| | | |
| | | |
⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -